Figure 8: a) Schematic Illustration of ion transport
in WS cell wall, and electricity generation via water transportation
through the micro/nanochannels. b) Potential profile in the EDL at the
NS/water interface. c) Comparison of Voc on DI water and
ethanol reservoirs. d) Effect of sealing on the output voltage. e)
Illustration of the overall integrated underlined mechanisms of the
high-performed WS-H+ device.
It is evident from the equation that the streaming potential Vs can be
very high by decreasing the channel diameter and decreasing the channel
length can give higher streaming current Is. The uneven diameter of the
micro/nanochannels of the NS structures creates an elevated pressure
difference (ΔP ) that facilitates achieving superior Vs and Is.
However, very narrow channels introduce a high flow resistance which can
be expressed by the Hagen–Poiseuille equation in Equation 5 . A
very high resistance may cancel the driving force from the capillary
pressure, preventing flow through the channel and resulting in a
substantially lower voltage and current output. Therefore, suitable
channel dimensions are essential for achieving the optimised higher
power output. Fine tuning of the device performance and channel
dimension is required.
The effect of concentrations, debeye length, and EDL overlap is
explained in the supplementary section, Equation- S 1. Water-evaporation-driven electricity generation has typically been
recognized entirely as streaming potential.[18] It
is difficult to differentiate these two processes because of the absence
of theoretical evidence and limited understanding of evaporation-induced
capillary pressure and evaporation potentials. A very recent
investigation focuses on illuminating the functions of evaporation
potential and streaming potential in the generation of
electricity.[19] Based on that point of view, in
this study, we explored the influence of evaporation potential through
the utilization of two different approaches. Initially, we substituted
the water with ethanol, which has lower zeta potential. The device
maintains the ability to produce an open circuit voltage of 254 mV,
depicted in Figure 8 (c). A decrease from the initial 601 mV
was observed with water as the medium. This implies that evaporation
plays a partial role in the whole procedure, as clarified by the
governing Equation- S 2 for the potential of evaporation
produced by ethanol. Additionally, sealing the device with an encloser
leads to a reduction in voltage output from 617 mV to 412 mV after 10
minutes, as it successfully minimized evaporation, portrayed inFigure 8 (d). These results emphasize the significant effect of
both processes and signify that a substantial part of the voltage
originates from the evaporation procedure.
This study aims to gather a full understanding of the reported events by
inspecting the effects of different chemical treatments on ion transport
and electricity generation. Whenever the WS-H+ samples
were independently subjected to DI water and OH-solutions, the chemical reactions took place, which enhanced the output
power. When H+ ions from the WS-H+approached to OH- ions of the solution, they
interacted spontaneously as per the equation H+ +
OH- = H2O. Therefore, continuous
movement and migration of H+ ions occur in the
acid-base process. As long as the top and bottom surface of the WS has
concentration variations (different functional groups), the
Voc and I sc show elevated values. The
differential ion flow causes an elevation in both open circuit voltage
and short circuit current, which is mostly due to chemical reactions and
partly due to underlying physical phenomena, as illustrated inFigure 8 (e ). To evaluate the effect of acid-base reactions, an
additional experiment was performed by soaking regular paper in acid and
placing it in an alkaline reservoir. The results revealed a substantial
voltage, as seen in Figure-S 7.
Power density analyses and
scalability for practical
applications
These WS-WEG devices can charge commercial capacitors of 47 µF, 470 µF,
and 1000 µF, reaching the stable voltage of 615 mV without any secondary
rectifier. Figure 9 (a) shows the required time to reach the
peak voltage by these three capacitors is 15 s, 110 s which is very fast
compared with the existing WEG devices [43], [60], [61].Figure 9 (b) shows the power density of the WS-WEG device with
the varied resistance from 10 Ώ to 108 Ώ. The highest
power density, 5.90 μW/cm2, of this WS-WEG device, was
observed with the external load value of
this WS-WEG device’s current-voltage (I-V) characteristics were
assessed. The Figure 9 (c) exhibits a linear reduction in
voltage while applying current. Despite the application of a negative
current, it is shown as positive for the purpose of simplicity. The
highest power density was consistently measured as 5.96 μW/cm².
Scalability is essential for the practical use of any devices.
Therefore, multiple devices were connected in series and parallel to
justify their performance. Figure 9 (d) illustrates the
Voc of 1.77 V while connecting four WS-WEG units in
series and Figure 9 (e) shows the Voc of 3.18 V
while connecting four WS-H+-WEG units in series.
Therefore, two WS-H+-WEG units connected in series and
two WS-H+-WEG units in parallel were able to power an
LCD-screened calculator without the aid of any external boosters or
rectifiers, as shown in Figure 9 (f) and S-V2. Figure 9
(g) depicts that these simple and innovative WS-WEG and
WS-H+-WEG devices show outstanding performance
compared with similar evaporation-driven WEG devices, which are based on
conventional organic and inorganic materials.